Highlights

Diamonds form from carbon that has been subjected to extremely high temperatures and pressures over long periods of time

Diamonds are found in kimberlite and lamproite “pipes” – the remnants of magma forcing its way to the Earth’s surface tens of millions of years ago

Diamonds can be extracted from underground mines, in open pits, or from alluvial deposits – sediments eroded from the diamond-bearing rock by flowing water

The fact that we even get to see these minerals is remarkable, when you consider the feats of nature that have to coincide to bring them anywhere near our reach. And let alone the human endeavour and ingenuity needed to find and mine them.

The story of any diamond begins deep within the Earth, and at least a billion years ago.

A matter of time

Diamonds form from carbon that has been subjected to extremely high temperatures and pressures over long periods of time. These conditions exist only at depths of 150 to 200km beneath the surface of the Earth, in a part of the planet known as the mantle.

The diamonds that we mine today typically formed around a billion years ago. To put that age into context, it means they are more than twice as old as life on dry land. The first terrestrial plants appeared in a period of geological time known as the Silurian, which began 443 million years ago, long after the diamonds you see in a jewellery store window were crystallising.

Some are considerably older – at the Diavik mine in Canada, diamonds can be more than 3.3 billion years old. The Earth is thought to be 4.5 to 4.6 billion years old, which means one of these diamonds could be three-quarters as old as our planet itself.

Explosive arrival

So why do we find diamonds near the surface, if they formed deep in the bowels of the Earth? It’s a dramatic part of their story, which geologists have only understood well in the last 50 years.

The rocks in which we find diamonds today were not the rocks in which they crystallised. Down in the mantle, beneath the level of diamond formation, molten rock – known as magma – formed. This magma forced itself upwards, melting through the diamond-bearing rock. Being resistant to high temperatures, the diamonds were preserved, and carried towards the surface.

Meanwhile, volatile compounds such as water and carbon dioxide dissolve in the magma, and as the pressure reduced near the surface, these volatiles turned into gas. This gas forced some of the magma to burst through the crust in explosive eruptions, while the rest was left to solidify in vertical pipe-like structures underground.

No human has ever witnessed this kind of eruption. Although these blasts occurred far more recently than the diamonds originally formed, they still typically happened tens of millions of years ago.

Kimberlite Kimberlite
Kimberlite

More than 12,000 kimberlite deposits have been found worldwide in the last 25 years, yet less than one per cent contained enough diamonds to be economically viable

Treasure hunt

Today, geologists hunt for these pipes, which are commonly of a type of rock called kimberlite. They look for “indicator minerals” – colourful grains of other minerals that weather out of kimberlites and are retained in sediments that can be traced back to their source. And they search for other tell-tale clues, like depressions in the topography, often covered by lakes, where kimberlites have worn away – being softer than the older surrounding rock in which they sit.

More than 12,000 kimberlite deposits have been found worldwide in the last 25 years, yet less than one per cent contained enough diamonds to be economically viable. In that tiny fraction, however, were the kimberlite pipes beneath Lac de Gras, Canada, which would become the Diavik Diamond Mine, in which Rio Tinto has a 60 per cent interest.

Trying a different tack, in the 1970s a geologist called Ewen Tyler set out to prove that another rock type – lamproite – could also bear economic quantities of diamonds. Tyler and his team were successful, and their discovery of diamond-bearing lamproite pipes in Western Australia eventually led to Rio Tinto’s other diamond mine – Argyle – being built.

Once enough diamonds are discovered to warrant a mine, that’s when the next chapter in their story begins. They can be extracted from underground mines, in open pits, or from alluvial deposits – sediments eroded from the diamond-bearing rock by flowing water. They are then processed, cut, polished and set into sparkling jewellery that preserves the prehistory and power of planet Earth. Find out more.

Under the microscope

Under the microscope

Diamonds are the only gems to comprise a single element: carbon. They may contain other elements in trace amounts which influence their colour or shape, but which are not part of their essential chemistry.

Diamonds are the only gems to comprise a single element: carbon. They may contain other elements in trace amounts which influence their colour or shape, but which are not part of their essential chemistry.

Another mineral, graphite, is also formed from carbon alone – but its properties could barely be more different. This black, soft, slippery mineral is perhaps best known for forming the “lead” of a pencil.

Although formed of exactly the same element, it’s the structure at an atomic level that explains the difference. In diamonds, each carbon atom is bonded to four others, in the same way in all directions, forming a stable, rigid lattice that gives it its hardness. In graphite, each carbon atom is bonded to three others. The atoms are arranged in layers with only weak forces between them – meaning they slide easily across each other.

Image: Diamonds are the only gems to comprise a single element.